1. Field of the Invention
The present invention relates to a poly (imide amic ester) random copolymer, a precursor thereof, and a process for preparing the same; more particularly, to a novel precursor of polyimide, poly (imide amic ester) which is chemically stable and has excellent work ability in either liquid or solid state, a polyimide obtained therefrom and a process for preparing the same.
2. Description of the Prior Art
Generally, polyimide is produced by the imidification of a poly (amic acid) as a precursor through thermal and chemical dehydration, which is prepared by the reaction of diamine and dianhydride.
The polyimide so produced is usually insoluble in an organic solvent (except for in rare cases) and mostly has neither fusing ability nor workability. When preparing polyimidic resin, usually the solution of the poly (amic acid) (a precursor of polyimide) is processed and subjected to imidification through thermal or chemical process to produce the polyimide which is then formed into the desired polyimidic resin.
However, the poly (amic acid) used as a precursor of polyimidic resin is often decomposed into an amine terminal radical and an acid anhydride terminal radical by depolymerization due to reaction equilibrium when it is stored in its liquid state, when it is being processed into a desired shape or when it is subjected to thermal imidification. As the result, the characteristics and quality of polyimide finally obtained is deteriorating and an offensive odor is produced due to the diamine compound produced as a result of the decomposition.
In addition, acid anhydride radical is formed in a small amount terminal as a result of the decomposition and will react with water flown in from outside or produced during the imidification and thus will be transformed into dicarboxylic acid which can no longer react with the amine terminal radical and the molecular weight of the poly (amic acid) will decrease over time when stored.
Such decrease in the molecular weight of the poly (amic acid) affects the characteristics of the polyimide produced therefrom. The product of the polyimide is undesirably deteriorating in its mechanical intensity and other chemical and physical properties.
Meanwhile, not many solutions which will hinder such depolymerization of poly (amic acid) have been reported:
The most frequently suggested method to suppress depolymerization of poly (amic acid) is to keep/store the solution of poly (amic acid), a precursor of poly imide at low temperature or to use the solution of poly (amic acid) shortly after its synthesis.
Schmidt et al. disclosed a process for preparing polyimide wherein a poly (amic ester) as a precursor instead of poly (amic acid). The poly (amic ester) is obtained by esterification of carboxylic acid which is the cause of the chemical instability of the amic acid group.
Thus, poly (amic ester) is a precursor having an improved chemical stability work ability (Macromolecules, 25, 6784 (1992)). The polyimide produced by this process has the characteristics of excellent stability and improved workability. However, the synthesis often tetracarboxylic acid diester dichloride which is an essential monomer for the preparation of poly(amic ester) is very troublesome and it reacts sensitively with nucleophilic compounds such as alcohol or water thereby being transformed into a compound which cannot be used as a monomer.
Thus, the tetracarboxylic acid diester dichloride is not only easily inactivated during storage but also has a strong toxicity which may cause many problems. In addition, in the solution of poly (amic ester) made by using such dichloride as monomer, chlorine produced in the reaction can exist in either ionic or molecular state, which may induce various kind of problem. Particularly, when polyimide is utilized for an industrial insulator which is one of its main utilities, fatal problems such as corroding electronic parts and deterioration of dielectric strength may be caused.
Meanwhile, Korean Patent Application No, 91-20171 and its corresponding U.S. patent application Ser. No. 07/930,376 filed on Aug. 17, 1992 and issued on Oct. 18, 1994 as U.S. Pat. No. 5,357,032, disclosed a poly(amic ester)formed by esterifying one half of acid radicals in a poly (amic acid) and a process for preparing aromatic polyimide-exchanged copolymerby imlidifingthe poly(amic ester) under heating. Since one half of the acid in the poly (amic acid) is esterified, the amount of free amic acid in the poly (amic ester ) produced is decreased by approximately 50% in comparison with that of a usual poly (amic acid). Accordingly, the poly (amic ester) has an increased stability in its liquid state.
In the same applications, an improved poly (imide amic ester) which is produced from the above invention and a process for preparing a poly (imide amic ester) which is a precursor of polyimide were also disclosed, in which the amic acid group of the poly (imide amic ester) is transformed selectively into imide radical by chemical imidification. Since the unstable amic acid group is removed, the stability of the polyimide exchanged copolymer solution is further improved.
However, the above two processes should be carried out through a complicated synthesis of a diamine monomer and the high polymer produced is restricted to polyimide-exchanged copolymer. Consequently, the above two processes cannot be used for the preparation of polyimide.
Under these circumstances, the inventions of the present invention strived to solve the above problems and discovered that when esterifying at least part of amic acid radicals of a poly (amic acid) which has been previously used as a common precursor of polyimide and than imidifying the remaining amic acid radicals that are not esterified, a poly (imide amic ester) random copolymer having coexisted ester and imide radicals is prepared.
The poly (imide amic ester) random copolymer has an improvement when in solution, can be used for the preparation of polyimide regardless of the structure of the poly imide and has many other advantages, when compared with poly (amic acid) a conventional poly imide precursor.
Since the poly (imide amic ester) prepared by the afore-mentioned process is very stable and soluble in solvents and has an excellent work ability as a solution, the poly imide resin prepared by thermal imidification of the poly (imide amic ester) preserved excellent characteristics which can be used in the electronic industry as a heat-resistant insulting high polymer material.